Reverse total shoulder arthroplasty (rTSA) is an innovative surgical procedure to return function to patients with severely debilitated shoulders. In 10 years of use in the US there have been significant advancements in implant design and surgical technique, and this evolution continues today. Advancements aside, there is still wide variability in patient outcomes that arise from their pathology, selection/placement of the implant, and the degree to which patient physiology compensates for the non-anatomic joint design. To increase functional range of motion (ROM) and reduce complications like scapular notching we must study the origins of their variability and create a benchmark against which to gauge future improvements. Also, we must generate prospective tools for clinicians to improve rTSA outcomes through modified surgical technique and implant selection. To do this we propose to: 1) Perform the first longitudinal analysis of recovery time-dependent changes in rTSA shoulder kinematics, 2) Integrate the first accurate model of dynamic 3D scapulothoracic/ humeral motion in a shoulder simulator, 3) Quantify the effects of humeral lengthening on external rotation ROM, and 4) Validate a prospective clinical algorithm to predict post-operative ROM in rTSA patients. The unique techniques outlined in this proposal provide a powerful toolkit to assess the impact of surgical intervention on the shoulder. In Aim 1, the research will differentiate the characteristic shoulder kinematics and implant configurations of patients with high and low ROM outcomes. The data will also lend insight into the onset of scapular notching as it relates to changes in scapulohumeral kinematics after rTSA. In Aim 2, a modular rTSA and dynamic scapula shoulder simulator will allow a kinematically accurate laboratory model of the shoulder to be used to investigate the relationships between humeral lengthening, joint center of rotation, and external rotation ROM. These data will be used to create a clinical algorithm that predicts post-operative ROM, which will be validated by predicting the degree of post-operative ROM in patients from Aim 1. Together these findings will benefit future rTSA patients as the goal of reducing variability and increasing achievable ROM is realized. The clinical ROM algorithm will give clinicians a new tool to prospectively assess the impact of rTSA implant component selection and placement on post-operative ROM. Kinematic data can be used to develop targeted rehabilitation protocols that stabilize the scapula and increase ROM in rTSA patients, or improve the accuracy of musculoskeletal models that examine motion and muscle forces in normal, pathologic and repaired/replaced shoulders. High accuracy bone/implant motion profiles can also be used to study rTSA phenomena like implant impingement/levering and joint instability, and precisely quantify skin marker motion artefacts common in shoulder motion analysis. Finally, the dynamic scapula simulator will create a platform to study not only rTSA, but any soft tissue repair or joint replacement using the normal, pathology- and repair-dependent motion of the shoulder as a baseline for comparison.